There are several places available to ethically own a Horsfield tortoise, or any tortoise for that matter depending on your living situation.
Sites available for rescue:
SSPCA Exotic pet section in Scotland. Animals will come with a pet health check, but usually no known history.
RSPCA Reptile search in the rest of the UK. Animals will come with a pet health check, but usually no known history.
The Tortoise Protection Group (TPG) is a non-profit group of volunteers whose aim is to promote higher standards of welfare for tortoises. Tortoises available for rehoming can be found on their website or their Facebook group.
Sites available for purchase:
The only experience I have with purchasing a tortoise is from the Tortoise Den but the TPG (above) have compiled a list of breeders in the UK.
Ethical Considerations of Breeders vs Practical Concerns of Rescuing
Usually a debate reserved for dogs, breeding vs rescuing is a concept applicable to almost any animal where pet ownership is common. Without wading into the ethical implications of the concept of pets and animal ownership as a whole, I believe good tortoise breeders and comprehensive rehoming programmes are mutually beneficial to one another in improving the quality of life for captive owned tortoises.
I will always advocate for rescuing as a primary option, below outlines some points to consider when making the right decision for you:
Lack of history with rescue tortoises can mean unknown health issues that appear later in life, or irreversible health issues. The most common examples with tortoises is Metabolic bone disease (MBD) due to lack of calcium, but found early enough can be mitigated with correct animal husbandry (calcium, UVB). On the contrary, you can only really be sure that you are purchasing a healthy tortoises when using a reputable breeder.
Without proper research, many who purchase tortoises don't appreciate their lifespan and eventual size, which I see most commonly with Sulcatas. Rescuing an older animal means they are likely fully grown so you can plan for size requirements, and naturally have shorter lifespans due to being older. This is not a luxury of a hatchling, so do your research.
Full disclosure, we purchased Melvin from The Tortoise Den, as we didn't realise we were able to rescue or re-home a tortoise. In hindsight, I wish we had rescued, but I can only commend how helpful the Tortoise Den were, and still are (note that this is not a paid endorsement). Some time later we adopted musk turtles from the SSPCA.